In a digital microwave system, microwave devices normally need to be installed outdoors, for example, mounted on a tower or pole. On some occasions, an operator needs to pay fees by area occupied. Therefore, the operator wishes to use fewer devices to transmit a larger amount of data. In the prior art, the solution to this problem is: using the dual-channel bonding technology in collaboration with the high-order quadrature amplitude modulation (QAM, Quadrature Amplitude Modulation) technology. This method saves devices and installation costs while improving the data transmission capacity, and therefore is popular among operators. The specific process of collaboration of the dual-channel bonding technology with the high-order QAM modulation technology may be understood by referring to FIG. 1. A first modulation signal in a modem 1 and a second modulation signal in a modem 2 at a transmitting end are combined and enter an antenna. The combined modulation signals are transmitted to a receiving end through the antenna. Before the first modulation signal is demodulated in a modem 1 and the second modulation signal is demodulated in a modem 2 at the receiving end, the combined modulation signals need to be split into two paths. The first modulation signal needs to be separated from the second modulation signal. After separation, the first modulation signal enters the modem 1 at the receiving end for demodulation, and the second modulation signal enters the modem 2 at the receiving end for demodulation. Although this dual-channel bonding technology satisfies the requirement for improving the signal bandwidth, adjacent channel interference will be caused when the modulation signal of two adjacent channels are bonded and then separated. To be specific, the first modulation signal cannot be completely separated from the second modulation signal at the receiving end, a part of the second modulation signal remains in the first modulation signal, and similarly, a part of the first modulation signal remains in the second modulation signal.
In the prior art, the method for solving the problem of adjacent channel interference is filtering the separated two paths of modulation signals by using a band pass filter (BPF, Band Pass Filter) before the separated two paths of modulation signals enter the modem 1 and the modem 2 at the receiving end, and designing a low pass filter (LPF, low pass filter) in the modem 1 and the modem 2 respectively to filter the received modulation signals before the signals are decoded.
In the prior art, the modulation signals are filtered by the band pass filter and the low pass filter before being decoded. However, the band pass filter's capability of filtering the modulation signals of the adjacent channels is quite limited. The interference signals of the adjacent channels are filtered by only the low pass filter in the modem. Therefore, a large number of adjacent channel interference signals may remain, which causes remarkable decrease of the signal to noise ratio.